This invention relates to an electric power meter for measuring an integrated electric power of an alternating current.
An electric power meter of the type described, is typically a watt-hour meter which measures an integral value, with respect to time, of an electric power of the alternating current. The alternating current has a frequency of, for example, 50 Hz or 60 Hz.
A conventional electric power meter comprises a first converter which converts a first input signal representative of an analog electric current of the alternating current into a first digital signal. A second converter converts a second input signal representative of an analog voltage of the alternating current into a second digital signal. An analog-to-digital converter is typically used as each of the first and the second converters.
Connected to the first and the second converters, a multiplier calculates a product of the first and the second digital signals to produce a product signal. Connected to the multiplier, an integrator integrates the product signal to produce a meter output signal representative of an integrated electric power of the alternating current.
With this structure, quantization noise or distortion is inevitably generated in each of the first and the second converters as known in the art. Generation of the quantization noise results in a degraded accuracy of measurement of the integrated electric power.
In order to elevate the accuracy of the measurement, it is necessary to use, as each of the first and the second converters, an analog-to-digital converter of a high accuracy of analog-to-digital conversion. Inasmuch as the analog-to-digital converter of the high accuracy has generally a complicated structure so as to produce an output signal having an increased number of bits, it is unavoidable that the conventional electric power meter has a complicated structure.
In addition, an offset is inevitably generated in each of the first and the second converters. As will later be described more in detail, the offset appears in an output signal of each of the first and the second converters and is equal to a difference between a desired output signal and an actual output signal which is actually obtained in the converter in question. Generation of the offset also results in a degraded accuracy of measurement of the integrated electric power.